The invention relates to a control system and method for controlling the phase difference between air/fuel ratio oscillations in first and second cylinder groups of an internal combustion engine.
Known internal combustion engines generally have first and second cylinder groups that are connected to either straight pipe exhaust systems or Y-pipe exhaust systems. Both types of exhaust systems further utilize catalytic converters to reduce engine emissions.
Known engine control systems utilize a measurement of exhaust gases from the first and second cylinder groups to maintain a predetermined air/fuel ratio in each of the first and second cylinder groups. Generally, the predetermined air/fuel ratio is stoichiometric. Further, when two state exhaust gas oxygen sensors are utilized, a typical result is a fluctuation, or oscillation, of the exhaust air/fuel ratio about the predetermined air/fuel ratio.
With engines having Y-pipe exhaust systems, known control systems have attempted to maintain a desired phase difference between first and second air/fuel ratio oscillations in the first and second cylinder groups, respectively, to improve performance of catalytic converters to reduce emissions. In particular, known control systems have attempted to phase shift one of the first and second air/fuel ratio oscillations either in phase or out of phase with the other of said first and second air/fuel ratio oscillations, to reduce emissions.
With engines having straight pipe exhaust systems and Y-pipe exhaust systems, known control systems have also delivered an air/fuel bias, either rich or lean of stoichiometry, to a cylinder group of an engine to reduce emissions. Those skilled in the art will recognize that during high engine speeds, a rich air/fuel bias delivered to a cylinder group can reduce NOx emissions from a downstream catalytic converter. Further, during low engine speeds, a lean bias (i.e., additional oxygen) delivered to a cylinder group can enable complete combustion of an air/fuel mixture to reduce HC emissions from a downstream catalytic converter.
The inventors herein have recognized that when known control systems phase shift the air/fuel ratio oscillations in a cylinder group, a desired average air/fuel bias in the cylinder group is changed. In other words, the average air/fuel bias is not maintained in a cylinder group when air/fuel ratio oscillations in the cylinder group are being phase adjusted. Thus, when this occurs, the emission performance of a downstream catalytic converters may be degraded.
Thus, the inventors herein have recognized, that a control system and method are needed which can phase shift air/fuel ratio oscillations in a cylinder group while maintaining a desired average air/fuel bias in the cylinder group.
The invention relates to a control system and method for controlling the phase difference between air/fuel ratio oscillations in first and second cylinder groups of an internal combustion engine.
As discussed above, a method for controlling phasing of air/fuel ratio oscillations in first and second cylinder groups of an internal combustion engine is provided. In the engine, a first air/fuel ratio in the first cylinder group is controlled to oscillate about a first predetermined air/fuel ratio at a first frequency, responsive to a feedback signal from an oxygen sensor downstream of the first cylinder group. The first predetermined air/fuel ratio corresponds to an average air/fuel bias either lean or rich of stoichiometry. Further, a second air fuel ratio in the second cylinder group is controlled to oscillate about a second predetermined air/fuel ratio at a second frequency, responsive to a feedback signal from an oxygen sensor downstream of the second cylinder group. The inventive method includes determining a phase difference between the first air/fuel ratio oscillations and the second air/fuel ratio oscillations. Further, the method includes adjusting a phase of the first air/fuel ratio oscillations so that both first and second air/fuel ratio oscillations are at a predetermined phase offset with respect to one another, while maintaining an average air/fuel bias.
A control system in accordance with the present invention includes a first sensor communicating with exhaust gases from a first cylinder group. The first sensor generates a first signal indicative of first air/fuel ratio oscillations in the first cylinder group. The control system further includes a second sensor communicating with exhaust gases from the second cylinder group. The second sensor generates a second signal indicative of second air/fuel ratio oscillations in the second cylinder group. The control system further includes an engine controller configured to determine a phase difference between the first and second air/fuel ratio oscillations responsive to the first and second signals. The controller is further configured to adjust a phase of the first air/fuel ratio oscillations so that both the first and second air/fuel ratio oscillations are at a predetermined phase offset with respect to one another, while maintaining an average air/fuel bias in the first cylinder group.
The inventive control system and method provide substantial advantage. In particular, the system and method can maintain a predetermined phase difference between air/fuel ratio oscillations in the first and second cylinder groups, while maintaining a desired average air/fuel bias. As a result, enhanced exhaust system performance is maintained even during phase adjustment of the cylinder groups.